While there are many patents related to sorting, relatively few are devoted to the specific peculiarities of sorting radioactive ore. My previous U.S. Pat. No. 4,194,634, Kelly, issued Mar. 25, 1980, reviewed several pertinent patents, namely Canadian Pat. No. 467,482, Lapointe, issued Aug. 22, 1950; U.S. Pat. No. 3,052,353, Pritchett, issued Sept. 4, 1962, and U.S. Pat. No. 2,717,693, Holmes, issued Sept. 13, 1955, and also explained the fundamentals of radiometric sorting as background to that invention. Other documents which are relevant to this topic are U.S. Pat. Nos. 3,011,634, Hutter et al; 3,075,641, Hutter et al; 3,216,567, Kelly et al; and 3,245,530, Kelly et al; and South African published application No. 78/3198, Hawkins et al (Sphere Investments Limited).
It is unnecessary to repeat that review, but it is important to emphasize two basic requirements of radiometric ore sorting: detection time and particle separation. While radioactive ores have the advantage of a built-in characteristic related to grade, i.e., radioactivity, this radioactivity is a random process and fluctuates greatly over a short period, obeying the Poisson Distribution Law. The longer the interval over which the radiation is measured, the greater will be the accuracy. To determine the grade of a piece of given size to a predetermined accuracy using a particular detector configuration requires that the number of counts detected in a given time fall within a given range of values. In addition to this limitation that detection is not instantaneous, but requires finite and appreciable time, radiometric ore sorting has the further problem that the pieces must be separated sufficiently, one from another, so that a detector is exposed to the radiations of only one piece at a time. Note that in configurations which use a plurality of detectors, no purpose is served by using "spaced apart" detectors. The pieces must be spaced apart, not the detectors. The only reason to separate the detectors physically is to allow the introduction of shielding material between, as required, but this by itself would be futile without separation of the particles.
It will be noted herein that the term "particle" is used to describe a rock or lump of ore regardless of size and is not intended to imply a particularly small piece. Thus, the terms "particle" and "rock" can be interpreted, for purposes of the present application, to be interchangeable.
The state of the art prior to the present invention includes two inventions of which I was a co-inventor which are described in U.S. Pat. Nos. 3,011,634 and 3,075,641, previously mentioned. These taught the use of gravity to achieve separation in free-fall past a single radiation detector, in combination with size determination and compensation. The above-mentioned Holmes U.S. Pat. No. 2,717,693 describes the use of multiple detectors in line under a horizontal conveyor belt, and accumulation of the counts from successive detectors. Pritchett U.S. Pat. No. 3,052,353 determines the mass or size of a radioactive ore particle as it moves longitudinally through a scanning head on a conveyor belt, with multiple scintillometers disposed around the belt in the scanning zone. Kelly U.S. Pat. No. 4,194,634 uses asynchronous movement of the particles, controlled by the radiation detected.
Finally, South African published application No. 78/3198 shows a plurality of spaced apart detectors under a conveyor as in the Holmes patent, in combination with a size scanner which may operate over the belt, but is shown in the drawings scanning the particles as they are projected from the end of the belt. The particles are shown spaced apart on the belt, but no mention is made of how they got that way, or that such separation is necessary. On the other hand, great importance is given to the detectors being spaced apart, but no reason is given why this is required.
In addition, there are the following types of radiometric sorters that have actually been manufactured, used and described in various technical papers or advertising brochures:
1. The Lapointe-patent type. Date about 1955. Detector under conveyor belt. No size compensation.
2. Holmes-patent type. About 1955. Multiple detectors under conveyor belt. No size compensation.
3. Hutter et al-patent type. 1958. Free-fall separation past size-scanner and single radiation detector. Size compensation.
4. Cotter Corp., Golden, Colorado, Sorting Plant type. 1975. High grade ore. Uses simple lines of spaced apart particles on conveyor belt with single detectors underneath. Incorporates size scanner and compensation. A paper by J. R. Goode published in "Proceedings of the 7th Annual Meeting of Canadian Mineral Processors" describes such units, and refers to provision of multiple detectors in line for lower grade ores.
5. Hawkins et al-patent application type. 1978-1979. Two almost identical uranium sorters have recently been developed by the only two manufacturers of radiometric sorters in existance, and are currently being sold in competition. These sorters were the result of teams of engineers working on the development which cost into the millions of dollars. The cost of a unit ranges from about one-half million dollars to well over 1 million dollars.
These two commercial sorters have the following features:
A channelized feeder and chute arrangement delivers particles onto a horizontal V-shaped top belt contacted by octagonal idlers which help to vibrate the particles into single lines, nose to tail. The lines of particles travelling horizontally at about 200-300 ft./min. are projected off the end of the belt, and separate as they fall and accelerate due to gravity. When their vertical speed reaches approximately 1000 ft./min. after a 4 to 5 ft. drop, they fall onto a slinger belt similar to the ones used for building mine waste dumps. The concave part of this belt converts the near vertical drop of the pieces to a horizontal movement at about 1000 ft./min., spaced apart, and now going in the reverse direction to the top belt.
The horizontal part of the slinger belt has a section for the rocks to roll around and settle, since they are now `standing on their head` compared to their stable position on the top belt. They then pass over multiple radiation detectors spaced apart in line under the belt, and fly off the end of the conveyor where they are scanned for size, and then either deflected or not by air blast.
The arrangement is mechanically complex and takes up a great deal of floor space.